1. Field of the Invention
The present invention relates to a non-resonant type knock sensor that is mounted on an internal combustion engine.
The invention also relates to a manufacturing method of such a knock sensor.
2. Description of the Related Art
A non-resonant type knock sensor mounted on an internal combustion engine is constituted so as to convert knocking vibration into voltage signals with a piezoelectric element sandwiched and press-held in an internal part of the internal combustion engine, and output the voltage signals as output signals when the knocking vibration takes place in the internal combustion engine. This type of knock sensor is well known by the Japanese Patent Publication (unexamined) No. 257624/2002, for example. FIG. 6 is a cross sectional view showing an internal structure of the knock sensor shown in the above publication. This knock sensor 40 is constituted as follows. A metal base 21 consists of a disk-shaped flange portion 21a and a tubular portion 21b axially extending from this flange portion 21a, and includes a through hole 22 provided through both of the mentioned flange portion 21a and tubular portion 21b. Onto this metal base 21, a lower-side insulating sheet 7, a lower-side terminal plate 5, a piezoelectric element 4, an upper-side terminal plate 6, an upper-side insulating sheet 8, and a weight 9 are put and fitted in sequence from the bottom. Next, a nut 23 is screw-engaged with a male thread part 21c that is formed on the outer circumferential surface of a tip end of the mentioned tubular portion 21b, and the mentioned components are fastened with a predetermined torque using a tool such as torque wrench fastening head. Thereafter, a terminal part 14 is joined to the lower-side terminal plate 5 and the upper-side terminal plate 6 by soldering or resistance welding, and the resultant structure other than an inner circumferential surface and two end faces of the mentioned tubular portion 21b of the base 21 is covered with a resin mold to form a case 13. Whereas, a connector part 15 for fetching out signals is formed protruding from one side face of the case 13 and is simultaneously molded with the terminal part 14 as an integral part.
This type of knock sensor is mounted on the internal combustion engine with a bolt to be inserted into the through hole provided in axial direction of the base. When knocking vibration takes place at the internal combustion engine, the components such as piezoelectric element, weight, etc. forming the knock sensor vibrate together with the knocking vibration, this vibration is converted into voltage signals with the piezoelectric element, and the detected signals are outputted to outside via the lower-side terminal plate and the upper-side terminal plate. Accordingly, it is necessary for all the components to be firmly pressed with a predetermined mechanical preload, as well as to be kept in this state at the time of occurrence of acceleration due to knocking of the internal combustion engine. Any change in preload causes output characteristics or detection sensitivity of a piezoelectric element to vary, thus making it hard to execute knocking detection.
Moreover, in the conventional knock sensor 40 constructed as described above, since the components such as the insulating sheets 7, 8, terminal plates 5, 6, piezoelectric element 4, and weight 9 that are stacked on the base 21 are sandwiched and pressed by clamping with the nut 23, there are problems as described below.
That is to say, although clamping with the nut 23 intends to provide a predetermined preload to the piezoelectric element 4, controlling a torque with a tool such as torque wrench fastening head, a problem exists in that a preload to be exerted on the piezoelectric element 4 is not stable due to fluctuations in clamping torque of the nut 23 or coefficient of friction or dimension between the female screw part of the nut 23 and the male thread part 21c formed on the base 21. This unstable preload causes fluctuations in output characteristics and detection sensitivity at the time of knocking detection. Furthermore, it is sometimes the case that metal chips are produced during machining a screw such as screw thread cutting, and the chips having been caught on the screw part drop and get in between electrodes of the piezoelectric element 4 at the time of sensor assembling or at the time of resin molding of the case 13 to bring the electrodes in short circuit eventually resulting in the disorder of the sensor.
Moreover, due to the fact that the nut 23 is used or that the male thread part 21c is formed on the base 21, not only a high production cost is required, but also it is necessary to pay attention to the control of a clamping torque of the nut 23 or the screw-engagement between the nut 23 and base 21. As a result, the conventional knock sensor has a further problem of making it difficult to achieve the automated assembling of a sensor.